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Triangle angle properties
Do the angles in a triangle always add up to the same thing? Would I ask it if they didn't? What do we know about the angles of a triangle if two of the sides are congruent (an isosceles triangle) or all three are congruent (an equilateral)? This tutorial is the place to find out.
Sal gives a formal proof that the measures of interior angles of a triangle sum to 180°.
Figuring out angles in a triangle. A little about exterior angles being the sum of the remote interior angles
Another example finding angles in triangles
Multiple ways to solve for the angles of multiple triangles
Interesting problem finding the sums of particular exterior angles of an irregular pentagon
Proof by contradiction that corresponding angle equivalence implies parallel lines
Example of angle hunting!
Showing a generalized way to find the sum of the interior angles of any polygon
More elegant way to find the sum of the exterior angles of a convex polygon